1. Field of the Invention
This invention relates to steel cords usable for the reinforcement of rubber article bodies such as pneumatic tires, industrial belts and the like. More particularly, it proposes an improvement in the steel cord of so-called compact structure composed of steel filaments in connection with developmental results for greatly enhancing the durable life of the rubber article by improving fatigue properties, particularly resistance to material fatigue and fretting wear, and strength retaining properties of the steel cord. It is particularly suitable as a reinforcement of pneumatic radial tires for truck, bus and light truck.
2. Related Art Statement
In a conventional pneumatic radial tire using steel cords as a reinforcement, the fatigue properties of the carcass ply and belt layer are degraded mainly by the following causes:
(1) Material fatigue due to repeated strain
It is a phenomenon that the material of steel cord is fatigued by subjecting the cord to repeated deformation during the running of the tire to vary the strain of steel filaments constituting the cord. This strain variate becomes conspicuous as the contact pressure (friction) between the filaments becomes large or the restraint on the movement of each filament becomes strong even if the deformation of the cord is the same, which brings about the promotion of material fatigue; and
(2) Fretting wear in contact portion between mutual filaments
This is due to the so-called fretting phenomenon. Additionally, there is sometimes caused a corrosion fatigue due to water penetrating from the outside of the tire. These fatigue factors considerably deteriorate the service durable life of the tire.
Heretofore, it has been considered that the penetration of rubber into the inside of the cord is primarily effective for enhancing the corrosion fatigue properties of the cord, and consequently there have been proposed many twisting structures for providing sufficient rubber penetration (, which are called as a rubber penetration structure). In such a rubber penetration structure cord, the rubber layer is interposed between the steel filaments, so that it is also considered to hardly produce the rubber between mutual steel filaments or a so-called fretting wear.
The penetration of rubber into the inside of the cord is easily achieved in a single twisting structure cord used in a belt layer of a radial tire for passenger cars, wherein each of the steel filaments can completely be covered with rubber.
However, in case of multi-layer structure cords such as a two or three layer structure cord as used in the carcass ply or belt layer in tires for truck, bus or light truck, it is very difficult to completely penetrate rubber into the inner layer of the cord.
When some of the steel filaments are not covered with rubber due to incomplete rubber penetration, the corrosion fatigue properties of the cord are not improved too much even in the rubber penetration structure.
In this case, it is necessary to make the helical radius of the steel filament large to provide a sufficient space between the steel filaments for obtaining complete rubber penetration. If it is intended to apply such a twisting structure (loose twisting structure) to the multi-layer structure cord, when the cord is pulled under tension, the setting of steel filaments becomes non-uniform and consequently it is difficult to avoid a fear that premature breaking failure is caused in certain portions of the filament due to the ununiform tension.
In the multi-layer structure cord, therefore, it is difficult to enhance the corrosion fatigue properties and strength retaining property (resistance to fretting) by rubber penetration into the inside of the cord.
On the other hand, U.S. Pat. No. 4,332,131 has proposed a cord obtained by twisting plural steel filaments having the same diameter in the same twisting direction at the same pitch or a so-called compact cord, and discloses that such a cord is advantageous in view of the productivity.
However, the inventors have made studies with respect to the fatigue properties and found that under the same filament diameter such a compact cord (hereinafter referred to as a normal compact cord) such as 1.times.12 structure is fairly inferior in the fatigue properties to the conventinal steel cord of 3+9 structure.
As to repeated bending, Japanese Patent Application Publication No. 44-18385 discloses a method wherein the steel filament for an outer layer is made thinner than the steel filament for an inner layer in order to equalize the fatigue strength of the steel filament between the inner layer and the outer layer. The cord disclosed in this article comprises a center core and an outer cover composed of at least one wire layer or layer of strands each containing plural wires. In this type of the multi-layer structure cord, the twisting pitch is generally different between the inner layer and the outer layer, so that contacting between the mutual steel filaments approaches a point contact and consequently the contact pressure between the inner layer and the outer layer increases. This is apt to increase the strain of the filament or produce the fretting. Therefore, even if the filament diameter in the outer layer is made thin, the improving effect with respect to the above phenomenon can not be expected This is because the thinning of the outer diameter of the steel filament in the outer layer can reduce the strain in the bending deformation as compared with the case of using the steel filament of the original diameter, but it cannot control the phenomenon of increasing the strain due to the interaction between the steel filaments.
Among the aforementioned multi-layer structure cords, the normal compact structure having the same twisting pitch in each layer forms a complete line contact in the steel filaments between the inner layer and the outer layer, so that the contact pressure between the inner and outer layers produced when pulling the cord is small. Thus, the friction between the steel filaments in the bending deformation of the cord under tension becomes small, so that it is anticipated that the strain produced in the filament and the fretting are small and the corrosion fatigue properties and strength retaining property are good.
In the usual 3+9 cord, gaps are opened in any portions between sheath filaments. On the contrary, in the normal compact structure, there is no gap between the mutual steel filaments in the outer layer or the sheath, while the gap is rather opened between the sheath and the inner layer or the core taking the ellipsoid in section of the steel filament into consideration. Hence, the steel filaments are arranged so as to collide with each other in the sheath. As a result, when tension is applied to the normal compact cord, the contact pressure between the core and the sheath is certainly small, but a large contact pressure is produced between the adjoining steel filaments in the sheath and consequently cracks grow from the contact portion between the adjoining steel filaments as a fretting nucleus to lead the breakage of the steel filament. This is why the corrosion fatigue properties of such a cord become inferior to those of the usual 3+9 structure cord.